It is well recognized in the art that a continuous metallic coating operation, considered generally, includes the steps of first preparing the surface of the strip to be coated to receive the molten coating metal, thereafter passing the base metal into a bath of molten coating metal, and finally finishing the applied coating. As used in this specification, the term "finishing" includes the steps of controlling, smoothing and solidifying the coating metal.
According to conventional practice, the controlling and smoothing of the molten coating metal is accomplished by the utilization of exit rolls which contact the molten coating metal on the surface of the strip as it emerges from the molten coating bath. The solidification of the coating metal which remains on the strip is accomplished by various expedients including a water quench, or the like.
It is recognized in the art that the use of exit rolls as described above does not give highly precise control over the thickness of the coating. In fact, maintaining consistency of coating thickness from edge to edge of a given strip is accomplished largely by manually changing the contour of the exit rolls during operation. This of course is an extremely difficult operation and requires highly skilled operators. In addition, this continual changing of contours of the exit rolls gives rise to a rapid rate of deterioration, requiring replacement of exit rolls on a relatively frequent basis.
It is also known that exit rolls will frequently leave a characteristic "tiger stripe" marking on the surface of the coating due to grooves in the rolls, or to the presence of dross and oxide picked up from the bath and adhering to the rolls.
Commercial experience with metallic coatings generally has established that the use of a mechanical finishing arrangement including, for example, exit rolls imposes a maximum speed limitation on the entire process. That is, it is impossible when using exit rolls to operate a coating line beyond a certain strip speed and still obtain a satisfactory surface finish.
Experience has also shown that the use of exit rolls creates additional problems in specific situations. That is, for example, in coating steel with terne or lead, it has been necessary to include up to 17% tin in the coating bath in order to properly wet the exit rolls. Since tin is not particularly desirable in the coating bath, and is a relatively expensive material, it would be highly desirable to develop a system which could successfully apply a low-tin terne coating.